1. Field of the Invention
The present invention relates to a method and an apparatus for forming a modified cross-section wire material, for example such as the wire material adapted to be used in production of piston rings for use in an engine, and, more particularly to a method and an apparatus for forming a modified cross-section wire material adapted to be used in production of oil-control rings for use in the engine.
2. Description of the Related Art
As the engine power and the engine speed in an engine increase, a stress imposed on a piston ring used in the engine increases. Due to this, the piston ring of a conventional type made of cast iron is replaced with a steel piston ring made of stainless steels or any other suitable special steels. Particularly, as for an oil-control ring used in the engine (hereinafter referred to as the “oil ring”), as shown in FIG. 7, the oil ring is constructed of a modified cross-section wire material, for example such as an H-shaped steel-like wire material and the like having a complicated construction. Further, the oil ring is required to be formed with an extremely high accuracy in mass-production.
In a conventional method and apparatus for forming such a modified cross-section wire material in mass-production of the piston rings: first, a round wire material is formed into a square shape in cross section; and, the thus square-shaped wire material is then subjected to a drawing process through a plurality of passes in non-powered four-roll turks-head stands arranged in tandem, each of which stands includes four forming rolls 7, 7′, as shown in FIG. 9. Further, such a drawing process is repeated many a time and oft, for example, three to five times, so that the square-shaped wire material is gradually formed into a finished-shape wire material best adapted for use in mass-production of the piston rings. As for the piston ring made of stainless steels or special steels, it is necessary to repeatedly perform an intermediate annealing process several times in the drawing process in order to straighten the wire material in the process. Particularly, the wire material made of high alloy steels requires both the drawing processes and the intermediate annealing process to be performed many times. This increases the number of the passes in production of a completed product of the wire material, and therefore increases the manufacturing cost of the product.
Consequently, in the art, there is a special need for a method and an apparatus for forming a modified cross-section wire material at low cost and with a high accuracy in dimension, wherein the method and the apparatus are capable of realizing a considerable reduction in cost, and a considerable reduction in each of the number of the drawing processes and the number of the intermediate annealing processes. However, in drawing the wire material by using the non-powered turks-head stands, a large drawing force is required to increase a reduction ratio of the wire material in cross-sectional area in each pass in each of the turks-head stands. When such a large drawing force is applied to the wire material, a large tension is applied to the wire material and extends the wire material along its length. When the wire material is formed into a finished wire material or product, for example such as one (shown in FIG. 3) assuming an H-shaped steel-like form in cross section and provided with a web portion 8 and a flange portion 9, the wire material is merely extended along its length and therefore reduced in its cross-sectional area. Due to such reduction in the cross-sectional area of the wire material, the thus drawn wire material often fails to sufficiently fill up a grooved pass opening formed between the forming rolls 7, 7′ of each of the non-powered four-roll turks-head stands (shown in FIG. 9) when subjected to such a large tension applied to the wire material. As described above, in the conventional method and apparatus, it is necessary to reduce the reduction ratio of the wire material in cross section in each pass in order to: decrease the tension applied to the wire material; and, therefore sufficiently fill up the grooved pass opening of the forming rolls 7, 7′ of each of the conventional turks-head stands with the wire material. This makes it impossible for the conventional method and apparatus to reduce the number of passes, wherein each pass is performed in each stand to gradually form the modified cross-section wire material, so that the passes are progressively faster than the one preceding.
As for means for solving the above problem inherent in the conventional method and the conventional apparatus for forming the modified cross-section wire material: it is necessary to increase a reduction ratio in cross-sectional area of the wire material passing through the grooved pass opening which is defined between the forming rolls of each of the turks-head stands. However, as described above, the forming rolls of each of the conventional turks-head stands are not powered. Due to this, it is necessary for each of the non-powered turks-head stands to perform its drawing operation of the wire material by using a capstan, which is capable of pulling the wire material passing through each of the conventional non-powered turks-head stands in the drawing operation. However, the use of such a capstan causes the wire material to be subjected to a large tension. Due to the presence of such a large tension in the drawing operation of the wire material, the wire material tends to be excessively extended lengthwise and often fails to sufficiently fill up the grooved pass opening of each of the turks-head stands, which makes it impossible to form the wire material with a high accuracy in dimension. Further, when the reduction ratio in cross-sectional area of the wire material is increased to realize a much more heavier pass in each stand, a crack often appears in a surface of the wire material due to its lengthwise extension under the influence of the large tension applied thereto by the capstan. This crack often results in almost full breakage of the wire material thus subjected to such a large tension.
Consequently, it is not possible to solve the problem by merely increasing the number of passes (i.e., the number of the turks-head stands) since the thus increased stands of the turks-heads merely increase the sum of tensions imposed on the wire material during the drawing operation, and therefore increase the risk of almost full breakage of the wire material. Due to this, in order to solve the wire-breakage problem, in the conventional method and the conventional apparatus for forming the modified cross-section wire material such as one shown in FIG. 3, it is necessary to divide the drawing operation of the wire material into three to five independent sub-drawing operations and further necessary to interpose an intermediate annealing process adjacent ones of these independent sub-drawing processes. Further, in the conventional method and the conventional apparatus using the non-powered turks-head stands, since a fill-up ratio of a flange portion of the wire material in the grooved pass opening of formed between the forming rolls of each turks-head stand is low, it is necessary to increase the wire material in size. However, such a large-sized wire material needs more drawing work, which increases the number of the intermediate annealing processes each to be performed adjacent ones of the independent sub-drawing operations in the conventional method and the conventional apparatus.